vdpa: introduce get_vq_dma_device()

This patch introduces a new method to query the dma device that is use
for a specific virtqueue.

Reviewed-by: Eli Cohen <elic@nvidia.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230119061525.75068-3-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>